118 PROCEEDINGS OF SECTION C. 



porphyry are very common ; the alteration from one to the other 

 being clearly noticeable. 



Mitchell also drew attention to this metamorphism of the 

 Bovvning conglomerates,^ remarking : "In some parts it has a very 

 homogeneous aspect, and can with difficulty be separated from the 

 metamorphic and porphyritic rocks of the locality." The con- 

 glomerate and porphyritic rocks have the same dip as the under- 

 lying shales, and form the highest beds of the silurian system of the 

 Yass district. 



As one approaches the summit of Bowning Hills, the quartz 

 porphyry changes suddenly into a spherulitic felspar porphyry of a 

 brownish grey colour, very close grained and hard. Weathered 

 portions show a very decided flow structure, which in some speci- 

 mens are more marked on account of pieces of felsitic material 

 altering the almost parallel lines of flow. Near the summit of the 

 main Bowning Hill, this felspar porphyry shows signs of bedding, 

 and is roughly columnar. It is certainly not intrusive, everything 

 pointing to its being a volcanic flow or series of flows. Its appear- 

 ance is very similar to some of the volcanic flows investigated by 

 Mr. L. F. Harper, F.G.S,^ at the Murrumbidgee, and may possibly 

 be an outlier of one of the Rhyolitic flows so common in his section,, 

 and which he looks upon as of Devonian age. 



Outside of the Devonian area of Yass I have never met with a 

 felspar porphyry of tliis nature, all the porphyries so mixed up 

 with the silurian sediments being either fine or coarse quartz 

 porphvries, totally different to the Bowning Hill lava. It is purely 

 local, forming as it does the cap of the Bowning Hills, and covering 

 an area about two miles long by half a mile in width. If it is' De- 

 vonian in age, its solitary occurrence, so far from the main mass of 

 the Devonian lavas, will need explanation. 



The Devonian sedimentaries of Yass, which are very extensive, 

 are separated from the Silurian by a thick bed of lavas and tuffs, ^ 

 and. having quite recently been described by Mr. Harper, need 

 not be dealt with in this paper. 



In the foregoing summary of the Yass beds, I have referred to 

 the belts of porphyritic rocks, as Nos. 1, 2, and 3. in order to keep 

 them distinct for the present from the Yass Porphyries of Harper, 

 which he states to be post- Devonian.* The mode of occurrence of 

 the No. 3 porphyry, between the Yass and the Hume Beds is against 

 the theory of its being post-Devonian, as everything points to its 

 contemporaneity with the Silurian sediments, being more in the 

 nature of a volcanic flow, a view strongly supported by its bedded 

 nature, and associated asit is with the various beds of tuffs and 



1 Mitchell, J. : "Geological Sequence of the Bowning Beds," Aust. Assoc. Adv. Set., Vol. I., 1887 



p. 294. 



2 Harper, L. F. : " Geology of he Murrumbidgee District, near Yass," Kec. Geo. Siir. N.S.W., 



Vol. IX., Pt. I. 

 SShearsby, A. J. : Loe. cit. Harper. Loc. cit. 



Harper, L. F.: I,oc.:i'., 



